The optical deflector is used to scan the direction of a light beam such as a laser beam by means of an optical element. It is employed by a wide range of applications in a writing system of a digital photocopier and laser printer and in a display system of a laser display and retina display, as well as in a barcode reader, laser microscope, laser processing apparatus and three-dimensional measuring instrument. A polygon scanner with a rotating polygon mirror (Examined Japanese Patent Application Publication No. H6-52196) and a galvanometer scanner for causing rotational vibration of a single mirror are utilized. Further, a thin resonant scanner based on the semiconductor manufacturing technique (wafer process) has been proposed.
The resonant scanner employs a combination of a torsion beam and a single mirror to form a torsional vibration system based on the restoring force of the torsion beam and the inertia moment of the mirror. A high deflection speed is made compatible with a great deflection angle by driving at the resonant frequency of this system. The resonant scanner is characterized by reduced energy requirements resulting from a small mass of the movable portion and a prolonged service life achieved by absence of a sliding portion such as a bearing.
The torsion beam of a resonant scanner is generally manufactured by etching or similar processing of a silicon or other plate-like material. If the torsion beam is subjected to a change in outer dimension due to expansion and shrinkage resulting from temperature changes, the restoring force is also changes. In the meantime, since the inertia moment is hardly changed by the fluctuation of the outer dimensions due to expansion and shrinkage caused by temperature changes, the resonant frequency of the system fluctuates as a result. Further, the resonant frequency may fluctuates due to a change in the spring constant resulting from cyclic fatigue of the torsion beam or a change in ambient conditions such as air resistance.
In the vicinity of the resonant frequency, the amplitude exhibits an abrupt change in response to the frequency of a drive signal. Thus, if the frequency of the drive signal is fixed, the amplitude of the resonant scanner greatly fluctuates due to resonant frequency fluctuation. To meet this situation, the drive signal frequency may be made to follow the resonant frequency fluctuation. This method can maintain the amplitude constant, but the drive frequency fluctuates. This causes difficulties, because of change in an aspect ratio, for example, in an application of a video display where the scanning is in synchronism with a two-dimensional data.
The following proposals have been made to keep the resonant scanner amplitude at a constant level by correcting the resonant frequency fluctuation. the first is a method of adjusting the resonant scanner temperature to a constant level using a heater (e.g., Japanese Registration. Patent No. 2711158). The second is a method of adjusting the tension at a constant level using a mechanism for pulling a torsion beam (e.g., Laid-Open Japanese Patent Application Publication No. H8-146334). The third is a method of using electric force to correct the amplitude fluctuation (e.g., Laid-Open Japanese Patent Application Publication No. H5-45603).
The method disclosed in the Japanese Registration Patent No. 2711158, however, is not practical because of poor response in temperature control and extra power that must be consumed for heating. Further, the method disclosed in the Laid-Open Japanese Patent Application Publication No. H8-146334 is also not practical because the mechanism for controlling the tension by elongation deformation is too complicated and difficult to adjust due to a high sensitivity to fluctuation. Further, the method mentioned in the Laid-Open Japanese Patent Application Publication No. H5-45603 is also not practical because adjustment is difficult due to abrupt and non-linear amplitude fluctuation, and extra power must be utilized.